2016 May

Novel terahertz source

Wednesday, 25th May 2016Publication highlights
Principle of operation of the emitter. An extremely short laser pulse initiates electron transport from the magnetic into the non-magnetic metal. Importantly, there are two distinct types of electrons which differ by their spin (thick light blue arrows) and their number (length of these arrows). Inside the non-magnet, the electrons experience a deflection which depends on the orientation of the electron spin. The resulting short charge current burst along the red arrow causes the emission of a terahertz pulse.

Principle of operation of the emitter. An extremely short laser pulse initiates electron transport from the magnetic into the non-magnetic metal. Importantly, there are two distinct types of electrons which differ by their spin (thick light blue arrows) and their number (length of these arrows). Inside the non-magnet, the electrons experience a deflection which depends on the orientation of the electron spin. The resulting short charge current burst along the red arrow causes the emission of a terahertz pulse.

International team of scientists realizes a compact, efficient and ultrabroadband emitter of THz radiation based on spintronic effects.

Berlin, 23. Mai 2016 – Terahertz waves offer numerous applications ranging from imaging tissue in medicine to airport security systems. However, the generation of such radiation by a low-cost source has remained challenging. Physicists from an international collaboration including the Fritz Haber Institute (Berlin), the Johannes Gutenberg University (Mainz), the Ernst Moritz Arndt University (Greifswald) in Germany and research institutes in France, Sweden and the United States have now realized a new concept for the generation of terahertz waves using spintronic metals. In contrast to previous designs, their emitters consist of thin metal films and take advantage of the spin rather than only the charge of the electron. Following this approach, they developed broadband emitters fully covering the 1-to-30-THz range, while at the same time being cost- and energy-efficient.

Terahertz radiation covers the frequency range 0.3 to 30 THz and is located between the microwave and infrared region of the electromagnetic spectrum. Many materials absorb THz radiation in a characteristic manner, while textiles and plastics are largely transparent. Unlike X-rays, terahertz radiation is harmless to biological structures. As a consequence, terahertz waves can be used for bio-imaging (such as in body scanners at airports), for quality control of food and for material identification.

One obstacle preventing the wide use of terahertz radiation is that current technology requires expensive and large lasers for broadband generation.

Photography of the prototype terahertz emitter.

Photography of the prototype terahertz emitter.

The terahertz sources fabricated by the researchers in Berlin, Mainz and Greifswald are scalable and can be used as table-top emitters (see figure). These novel spintronic emitters cover the complete range of terahertz frequencies from 1 to 30 THz without gap. Key features are higher energy efficiency, lower fabrication costs and easier operation.

The new terahertz emitter resembles a photodiode or a solar cell: Upon illuminating the material with an ultrashort laser pulse, a current burst is created (see figure). Consequently, this current burst radiates an electromagnetic pulse with frequencies in the terahertz range, similar to an antenna. In contrast to a solar cell, the metal films are only 5.8 nm thick, thereby ensuring that the current burst is extremely short, while at the same time preventing attenuation of the terahertz wave inside the emitter. By systematically optimizing the emitter materials and their thicknesses, relatively weak laser radiation from compact laser sources is now sufficient to generate the entire spectrum from 1 to 30 THz.

The performance of the emitter is greatly enhanced because the researchers also took advantage of the spin of the electron, in addition to its charge. The spin is a magnetic property of the electron that changes the motion of electrons when flowing through a magnetic metal. In the new emitter, this effect is used to steer the motion of electrons such that they can emit the terahertz wave in a particularly efficient manner.

To optimize the emitter performance, the scientists had to screen a large number of materials with varying material composition and geometry. The high-throughput Rotaris sputter deposition system installed at the Institute of Physics at Mainz by Singulus Technologies was a crucial prerequisite to fabricate a large number of samples in a short time. The optimization procedure was further supported by calculations of theorists at Forschungszentrum Jülich.

Original paper:
Efficient metallic spintronic emitters of ultrabroadband terahertz radiation;
T. Seifert et al., Nature Photonics, http://dx.doi.org/10.1038/nphoton.2016.91

Contact:
Tobias Kampfrath, Terahertz Physics Group, FHI
kampfrath@fhi-berlin.mpg.de